Generally a wind turbine converts energy in the wind to electrical energy supplied to a utility grid.
As wind acting on a wind turbine rotor produces rotational torque in the drive train of the wind turbine and, the rotor of the wind turbine is influenced by varying wind situations e.g. due to wind shear, alternating mean wind velocities, fluctuating wind, turbulence etc., the rotational speed of the drive train and hereby also the generator varies accordingly. This in turn have an influence on the power quality produced by the wind turbine generator. Furthermore the varying rotational speed may produce a varying mechanical torque in the drive train that can have fatigue influence on mechanical components of the drive train.
US patent application 2006/0066111 discloses a wind turbine vibration damping system that control the torque in the drive train produced by a wind turbine generator, based on information regarding the rotational speed of the generator.
One problem related to this system is that frequencies of the wind turbine, which are desired to dampen, are calculated based on Fourier transformations of a sampled signal indicative of the rotational speed of the generator. A problem related to this problem is that continuous Fourier transformations and calculations are time consuming and takes up computational power of the wind turbine controller and due to time delay the accuracy of dampening is therefore weakened.
It is an object of the present invention to provide an advantageous method of dampening varying rotational vibrations in the drive train of a wind turbine without the above mentioned disadvantages.